QtIFW

Fix broken start menu directory handling.

While leaving the page, the value was updated in the core engine,
but once the user returned back and choose a new one from the list
it was set to the selected directory plus the updated one.

Task-number: QTIFW-559
Change-Id: I907fd789bd4460b7479b4f17a06e3d31a8d22c20
Reviewed-by: Niels Weber <niels.weber@theqtcompany.com>
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>